Generally, a piston type compressor for use in an automotive air conditioning system comprises a cylinder block having a plurality of cylinder bores. A plurality of pistons are slidably disposed in the respective cylinder bores and reciprocated by, for example, a swash plate or wobble plate in the cylinder bores. In a variable capacity swash plate type compressor with a mechanism varying an inclination angle of the swash plate, a single-headed piston is generally used. The single-headed piston includes a body with a head, and a support portion for receiving shoes which convert rotation of the swash plate into reciprocation of the pistons. However, a bending moment acts on the pistons due to a component of the force that is exerted normal to the direction of motion of the pistons during operation of the compressor. Accordingly, the bending moment causes the deformation of pistons, and thus, a contact portion between the pistons and the cylinder bores is abraded.
In order to clarify the problems occurring in a typical swash plate type compressor with a variable displacement mechanism, description will be made with reference to FIG. 1. The compressor 1 of this type has a cylinder block 2, with a plurality of cylinder bores 4, and with front and rear ends of the cylinder block 2 sealingly closed by front and rear housing portions 6 and 8, respectively. The cylinder block 2 and the front housing 6 define an air-tight sealed crank chamber 10. A valve plate 12 is mounted between the rear end of the cylinder block 2 and the rear housing 8. The rear housing 8 has formed therein inlet and outlet ports 14 and 16 for input and output of a refrigerant gas, a suction chamber 18, and a discharge chamber 20. The suction and discharge chambers 18 and 20 are in communication with the respective cylinder bores 4 via suction and discharge valve mechanisms. A drive shaft 22 is centrally arranged to extend through the front housing 6 to the cylinder block 2 and rotatably supported by bearings mounted in the front housing 6 and the cylinder block 2. The cylinder block 2 and the front and rear housing 6 and 8 are combined by screws 25. A rotor 26 is mounted on the drive shaft 22 in the crank chamber 10 to be rotatable with the drive shaft 22, and is supported by a thrust bearing 28 seated on an inner end of the front housing 6. A spherical sleeve 30, having an outer spherical surface formed as a support surface, is slidably supported by the drive shaft 22. A spring 32, mounted around the drive shaft 22, is interposed between the rotor 26 and the spherical sleeve 30, and biases the spherical sleeve 30 toward the rear housing 8.
A swash plate 34 is rotatably supported on the outer surface of the spherical sleeve 30. The swash plate 34 is connected to the rotor 26 via a hinge mechanism so as to be rotated with the rotor 26. The hinge mechanism includes a support arm 36 that protrudes axially outwardly from one side surface of the rotor 26, and an arm 38 that protrudes from one side surface of the swash plate 34 toward the support arm 36 of the rotor 26. The support arm 36 and the arm 38 overlap each other and are connected to each other by a pin 40. The pin 40 extends into a pin hole 42 formed through the support arm 36 of the rotor 26 and a rectangular shaped hole 43 formed through the arm 38 of the swash plate 34. In this manner, the rotor 26 and the swash plate 34 are hinged to each other, and the sliding motion of the pin 40 within the rectangular hole 43 changes the inclination angle of the swash plate 34 so as to change the capacity of the compressor.
Pistons 44 are slidably disposed in the respective cylinder bores 4. Each piston 44 has a body 46 with a head portion which is slidably disposed in the corresponding cylinder bore 4, and a bridge portion 48 which has formed therein a recess 50. Semi-spherical shoes 52 are disposed in shoe pockets 54 formed in the bridge portion of the piston 44 and slidably engaged with a peripheral portion of the swash plate 34. Therefore, the swash plate 34 is rotated together with the rotation of the drive shaft 22, and the rotation of the swash plate 34 is converted into the reciprocation of the pistons 44.
A cutout portion 56 is formed at a lower front end portion of the piston 44 to prevent contact between a side surface of the swash plate 34 and the body 46 of the piston 44 when the piston is in its bottom dead center position.
A control valve means 60 is provided with the compressor to adjust a pressure level in the crank chamber 10.
In the above-described type of compressor, a bending moment generated from various forces acting on the pistons 44 causes a deformation of the pistons 44 and potentially excessive abrasion about a contact portion between the pistons 44 and their corresponding cylinder bores 4. FIG. 2 illustrates an enlarged partial view of FIG. 1, showing the various forces acting on a piston. During the compression stroke of the piston 44, the pressure P.sub.c in the crank chamber 10 acts on the forward end of the piston 44 while a compression reaction force P.sub.d acts on the other end of the piston 44. The pressure P.sub.c in the crank chamber 10 and the compression reaction force P.sub.d act on the swash plate from the piston via the shoes 52 creating an action force on the swash plate 34, with obviously a reaction force that is equal in magnitude and oppositely directed to the action force. That is, when the piston 44 is in its compression stroke, the force F exerted from the swash plate 34 on the piston 44 acts in a direction that is perpendicular to surfaces of the swash plate 34 at a contact location where the semi-spherical outer surface of the shoe adjacent to the body of the piston 44 comes into contact with the semi-spherical inner surface of the shoe pocket 54. This location is at an apex of the shoe pocket 54 lying on the central axis O of the piston 44. If the force F exerted from the swash plate 34 on the piston 44 is decomposed into two components, a horizontal and a vertical component, there will be a horizontal component F.sub.x lying on the central axis O of the piston 44 and a vertical component F.sub.y being perpendicular to the central axis O of the piston 44. Let "m" be the mass of the piston 44, "a" the acceleration of piston during the compression stroke, and "A" the surface area against which the pressure acts. Thus, EQU .SIGMA.F.sub.x =ma EQU .SIGMA.F.sub.x =AP.sub.c -AP.sub.d +F.sub.x
By combining the above equations, we can write, EQU F.sub.x =ma+A(P.sub.d -P.sub.c)=ma+(.pi./4)*d.sup.2 (P.sub.d -P.sub.c)
and EQU F.sub.y =F.sub.x tan .theta.=tan .theta.[ma+(.pi./4)*d.sup.2 (P.sub.d -P.sub.c)]
which d is a diameter of piston.
The vertical component F.sub.y, then, will act on the piston 44 to create a bending moment which is maximum at the lower back edge designated by "p". As stated above, the cutout portion 56 is provided to prevent the piston 44 from coming into contact with the rear surface of the swash plate 34 when the piston 44 approaches its bottom dead center position. However, the cutout portion 56 creates a horizontal distance x between the operating point of the force F acting on the piston and the location of the reaction force acting on the cutout portion 56 at p. This distance x creates a bending moment which acts on the piston 44. The maximum bending moment M.sub.max acting on the piston is given by EQU M.sub.max =xF.sub.y =xtan .theta.[ma+(.pi./4)*d.sup.2 (P.sub.d -P.sub.c)]
Therefore, due to the bending moment, the piston will tend to cock in its cylinder in a counterclockwise direction with respect to the reaction force-operating point P, creating the possibility of abnormally excessive abrasion on the body of the piston about the reaction force-operating point P and in an edge portion diagonally opposed thereto.